Rog,
I replaced my crossmember just over a year ago! I took several measurements then chopped the old one out But on assembly nothing seemed to have moved so I tacked it into place then re-checked it & all seemed well so it was welded in & all turned out ok in the end
The job took most of a day to strip it all out & a day & a bit to replace & put all the other bits back on
It is quite a straight forward job but I think the key is to take your time & don't rush!
Good Luck,